When producing impact copolymer (ICOP) products, e.g., a propylene impact copolymer, a second reactor (also known as the impact reactor) is used in series with a first reactor. In the first reactor typically polypropylene homopolymer, known as the “matrix” material, is produced. In the second reactor (i.e., impact reactor), both propylene and comonomer(s), such as ethylene, are added to produce a “rubber-phase” dispersed within the matrix material. The resulting material is known as an impact copolymer, and it has beneficial properties, such as good impact resistance while maintaining much of its stiffness modulus. The weight fraction of the rubber material in the final ICOP product is called the fraction copolymer or rubber content, abbreviated Fc.
To increase the impact resistance of an ICOP, those skilled in the art know that the rubber content of the ICOP must be increased. The product in the second reactor is typically in the form of a powder and a major problem in producing high impact copolymer products (HICOP), e.g., with Fc greater than or equal to 25 weight percent (wt %), is particle stickiness. When the Fc exceeds a certain value (the value is a function of, among other things, reagents, catalyst and reaction conditions), the rubber material causes the surface of the powder particles to become sticky, particles agglomerate, and the continuity of the reactor operation can be compromised. In addition, particle stickiness is detrimental to product flowability, and so can cause problems in downstream operations such as flow through a purge bin or flow to a pelleter.
With certain catalysts, e.g., catalysts comprising a procatalyst, cocatalyst and a mixed external electron donor (MEED), and particularly for high rubber content HICOP, it is necessary to feed additional cocatalyst (also known as an activator) to the impact reactor in order to produce the needed amount of rubber material to achieve the desired Fc. This additional cocatalyst, e.g., triethylaluminum (TEA or TEAl), can also be problematic because it tends to increase the stickiness and reduce the flowability of the powder. Without being bound to a theory, the rubber material on the external surfaces of particles is believed to contribute to the stickiness and the addition of cocatalyst to the second reactor preferentially causes production of the rubber phase on the surface of the ICOP particles rather than within the interior of the particles.